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    输入两个字符串<span>S</span>和<span>L</span>，都只包含英文小写字母。<span>S</span>长度<span>&lt;=100</span>，<span>L</span>长度<span>&lt;=500,000</span>。判定<span>S</span>是否是<span>L</span>的有效字串。<span></span>
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        判定规则：<span>S</span>中的每个字符在<span>L</span>中都能找到（可以不连续），且<span>S</span>在Ｌ中字符的前后顺序与<span>S</span>中顺序要保持一致。（例如，<span>S=&quot;ace&quot;</span>是<span>L=&quot;abcde&quot;</span>的一个子序列且有效字符是<span>a</span>、<span>c</span>、<span>e</span>，而<span>&quot;aec&quot;</span>不是有效子序列，且有效字符只有<span>a</span>、<span>e</span>）<span></span>
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